1. Field of the Invention
This invention relates to test fixtures for applying loads to a rotary mechanical system and more specifically to text fixtures for applying a dynamic lateral load and isolating the measurement of the angular displacement of the rotational output under load.
2. Description of the Related Art
The use of rotary mechanical systems to power automobiles, drive robotics, actuate flight control systems on airplanes and missiles and many other mechanical systems is ubiquitous throughout our economy. The use of a motor to rotate a shaft to actuate these various systems is a cost effective and reliable way to convert electrical energy into a mechanical force. In many applications such as found in an automobile, the motor rotates the drive shaft at a high and relatively constant rate. Therefore, the shaft has a large range of motion but a relatively small frequency content. In other applications such as found in an airplane, the motor rotates the drive shaft over a small range of motion, less than ten turns or even a single turn, to actuate flight control. In applications such as found in a missile, the motor rotates the drive shaft over a small range of motion but at a very high rate to control the position of the fins, canards or wings to guide the missile.
The different applications and environments produce a wide range of load conditions for the rotary mechanical systems. Before a design can be approved or, in some cases, a particular system fielded, it must be tested to determine how the system performs under certain load conditions. More specifically, when a torque is applied to the shaft how does the system respond? When a lateral load is applied to the shaft with and without torque how does the system respond?
Conventional techniques for testing control actuation systems (CAS) used for steering control of guided missiles and flight vehicles typically employ fixed end torsion bars to simulate aerodynamic loads encountered in flight. As shown in FIG. 1, a CAS 10, referred to as a unit under test (UUT) during testing, includes an actuator 12 such as a motor and a drive shaft 14 that rotates about an axis 16 and rests on test bench 18. A conventional test fixture 20 includes an interface bracket 22 that is bolted to the end of drive shaft 14, a torsion bar 24 that is rigidly mounted on the interface bracket along axis 16, and a plate 26 that fixes the other end of the torsion bar to a mechanical ground. An encoder 28 (rotational sensor) measures the angular rotation of the torsion bar when the UUT is activated. The amount of torque in the torsion bar, hence the load applied to the UUT is proportional to the angle of rotation. Alternately, a torque sensor may be placed in-line between the torsion bar and drive shaft.
This approach limits the evaluation to linear load application and prohibits testing of the CAS under desired acceptance test procedures and realistic load environments demanded of typical flight scenarios. Specifically, a “torque at rate” test procedure requires the application of a constant torque load for a constant rotation rate of the drive shaft. Typical flight scenarios produce rapidly changing nonlinear load conditions. Clearly a fixed end torsion bar cannot replicate these conditions. To test the UUT over a range of load conditions albeit quasi-static an operator must replace the torsion bar with a different torsion bar having different stiffness properties. This is very inconvenient and slow.
A static lateral load may be applied through a load bearing 30 about the interface bracket 22 or torsion bar 24 via a mechanical actuator 32 such as a ballscrew, jackscrew or hydraulic system. In some cases, a force sensor is used to monitor the applied force and feed it back to a servo motor to maintain the desired static set point load. The bandwidth of such control systems is very low, <1 Hz, and marginally adequate to maintain the desired set point. To test the UUT over a range of static conditions, the operator must reprogram the actuator for each new value once the previous test is completed, which is inconvenient and slow. Furthermore, desired test procedures and actual flight conditions require dynamic time-varying loads, which are not supported by the current testing platforms. Moreover, the application of the lateral load to the shaft may impart a rotation on the encoder relative to the shaft that corrupts the measurement of the rotation angle. Furthermore, the load may be transferred to the encoder potentially damaging it.